Lathe jumps out of gear

I've had my Chester 12x36 geared head lathe for about 20 years, with no problems until now. I was turning a bit of aluminium when there was a clattering noise and the spindle stopped turning. The motor turned fine, but not the spindle.

I took the gearbox cover off and noticed that a gearwheel cluster wasn't in line with the corresponding gear on the spindle. See attached photo. The cluster revolves fine but wasn't turning the spindle.

Further investigation revealed that the gear cluster can move sideways by itself, without the turning the control knob. I think there should be a detent to hold it in any one of its 3 positions. The knob, the shaft, the fork, and the gear all move as they should, but when the knob is released there is nothing to maintain its position and the gear can drift sideways and disengage the drive.

I removed the two grub-screws in an attempt to remover the knob and discovered that there is a spring behind the left-hand one (not sure about the right-hand one). And the knob didn't come off - it must be the roll-pin that retains it. I figured that adjusting the grub-screws would do the trick, but the only effect it had was to restrict the full movement of the gear. I haven't tried removing the roll-pin.

Any ideas?

Gary,

Until you can get to see what is actually behind the control knob you don't know the full story. I would expect a ball bearing detent of some kind.

Regards Brian

I would second some balls under the springs, do you still have the manual? parts diagram may show what should be there and could be something a simple as the grub screws coming loose over time so there is no longer enough pressure on the balls

Hi Gary,

I am not a user, but I would guess there is a ball bearing in front of each of those springs, one of which should slip into a slot for each position of the gear. I’m guessing that if the gear ‘locks’ in one position, but allows it to float in the other, one detent (ball?) is stuck or worn, or the spring is broken.
I suggest you fish out those springs and see if the ball, or whatever is behind them, is to specification. If not it looks like removal of that roll pin will be necessary to determine any damage to the casing.

If the gear cluster can move sideways without moving the handle, it sounds as though the there is a disconnect between the handle and the shifting fork, or between fork and gear cluster. If you move the gear cluster by hand does the cross shaft rotate? Does the shifting fork move?

A few pictures from different angles might help. I think the shiny cross-bar is hiding the forks that move the gears. A few shots of the fork mechanism might reveal all.

For example, the two spring adjusters may be for fine adjustment after the fork has been set about right first, and after 20 years the shift mechanism has shifted or worn enough to take the coarse set-up out of range. If so, the coarse setting needs to be reset, probably by centring the gears in position 2, and then tweaking the fine adjusters as necessary to shift cleanly from 1 to 3. How exactly does the fork connect to the knob?

Dave

There might also now be wear-grooves between the locking dimples, exacerbating the effect of sticking or relaxed springs or a worn detent.

The ABC lever looks to be identical to the 123 lever, so if that is working correctly, you have something to compare against.

Looking at the photo, there are two grub screws with lock nuts on the shaft. I think that releasing these would allow the shaft to come forwards out of the headstock without beating on the roll pin. Hold a rag under the lever as you withdraw it or springs and balls might disappear into dark corners.

It could be that the rear, red one of these is slipping or ratcheting on the shaft so that is worth examining.

My Smart & Brown 1024 has similar balls and spring arrangements to give detents on the basic movements.

When I got it the springs were well mashed up so the detent system didn't operate making it very hard to select the gear required. New springs fixed it. No significant wear on the casting between the detent holes and no evidence of it having been disturbed and damaged during re assembly.

I can only assume that such spring mash-up things can "just happen".

I would have expected something a little more positive than simple balls and springs to keep a gear engaged tho'. The S&B lever has to be pushed down to unlock things.

Clive

I've investigated much closer. I removed the grub-screws, springs, and balls from the selector knob and removed the knob from the shaft. Neither the springs nor the balls showed any signs of damage or wear. The detents in the gear housing also looked fine. I made a cardboard template that fitted on the selector shaft and contained holes that revealed the detents in the housing. The following photos shows the detents and the cardboard template.

I also took a photo of the selector knob showing the positions of the holes for the two spring loaded balls.

I then found a short length of bar that fitted into the rear of the selector knob, and positioned the cardboard template on it to give a view as if looking from within the gearbox, outwards to the rear of the in-place selector knob. The three holes in the template representing the the three detents. I was then able to see how the detents aligned with the balls when the knob was rotated in its three positions.

I position 1, the detent at 9 o'clock lines up nicely with the hole in the knob. In position 2, the detent at 3 o'clock lines up nicely, but in position 3, when the detent at 8 o'clock is aligned with the knob hole, the 3 o'clock detent is nearly aligned with the other knob hole, which has the effect of both detents trying to align the shaft.

Because of this slight misalignment, the gear in the following picture gently drifts to the right.

Seems like a manufacturing fault to me. In the meantime I intend to adjust the fork to the best compromised position. Any ideas about a solution?

***EDIT*** There was no noticeable wear on the fork.

Edited By Gary Wooding on 29/12/2022 12:15:08

Hello Gary,

I might be tempted to bore out the rogue hole, fit a slug of metal to fill it and re-machine that detail to correct the situation. You can then reset the fork to the original position.

Regards Brian

Edited By Brian Wood on 29/12/2022 12:30:52

Looking at the pictures,both the forks, and the lever hub appear to be located on the shaft by grubscrews with locknuts.

Are both these tight and firmly in place?

On my lathe, a Far Eastern, belt driven head model, seemed to have a problem with belt slip. The actual problem was found to be that the dimple in the shaft for the clamping grubscrew was not deep enough, allowing the lever to "float".

Having deepened the dimple, the problem has not recurred.

Howard

Whoa up chaps

Original description stated that the lathe has been operating successfully for 20 years. Then suddenly we have a wandering gear selector. This isn't a manufacturing fault it's wear and tear.

Don't go modifying anything just yet, there's something as yet unexplained.

Rgds Simon

Gary, +1 for what Simon says above. Looking at your pictures the selector lever has 3 positions, with the holes as they are the detents will engage in 3 positions, if you alter then spacing as suggested then the detents would only engage in 2 positions.

Look further into the situation before you even THINK of modifying anything.

Please look very well at the photos he has posted and reconsider your advice.

In one gear, the left hand hole is used and lines up correctly. In the second gear, the right hand hole is used and lines up correctly. In the third gear, the left hand hole is used (again) and lines up correctly but half of the right hand one is showing.

So which one do you suggest he fills? If it is the left hand one, he looses two out of three gears. If it is the right hand one he looses one out of three gears.

+1 for Simon's thought about 20 yr operation vs problems now occurring. Suggest looking at shaft float on the selector shaft - is the rear bush on the headstock worn, allowing excessive knob / detent free play? maybe worth trying a shim at the rear to take up play.

Forgive me if I'm being thick, but the cardboard template with three holes is mimicing the stationery part, you need a piece of card with two holes the same as the two in the selector lever hub. The the one with two holes slides over the (stationery) one with three holes.

There's got to be three unique positions of the detents else it would never have worked from new. The stuff about a detent covering half a hole doesn't make sense.

Have you dismantled the selector cross rods and forks to check that there isn't a flat or a hole that's smeared and is sticking but moving erratically?

Keep us posted, do

Rgds Simon

Gary,

You've never said the control knob was loose only that the gear drifts, nor that the arm started pointing to the wrong sector.

Therefore it's one end of the selector shaft or the other that's loose. First thing I would have done is hold the fork and try to move the selector. Like your Centec there should be no sideways forces produced by the gears though I suppose if the shafts are not quite parallel that might cause it.

I did only say I might be tempted to carry out surgery, honest!

Maybe it would do no harm if Gary removed the ball and spring selectively from one hole at a time to see if the location 'conflict' resolves itself by doing so.

Retiring to my bunker now

Brian

Re reading the O P; the gear cluster can move sideways without moving the control lever

This suggests

1 that either the fork is worn, so that it no longer locates the cluster precisely enough, or

2 that the fork is not firmly secured to the control shaft, or

3 that the control shaft is not firmly secured to the control lever hub.

Howard